When Republicans need to get inside the brain of a Democrat, they call Rob Portman.

"I don't know anybody who's done more debate coaching or surrogate work in debates than Rob Portman," Rick Lazio, the former New York congressman who ran against Hillary Clinton for Senate in 2000, told Yahoo News. Lazio asked Portman to play his opponent in that race, and he left shocked at how well Portman could transform himself into the former first lady.

"No wig, no dress," Lazio said, "but otherwise very effective."

For nearly 20 years, Republicans have turned to Portman, now a senator from Ohio and widely rumored to be near the top of Mitt Romney's list of preferred running mates, for help with debate preparation. The freshman senator has a natural talent for throwing himself into the role of the opposing candidate in mock debates. According to people who have seen Portman perform this feat, he nails it every time.

Since 1996, when Bob Dole tapped Portman to channel Lamar Alexander in a practice debate, Portman has played the role of Hillary Clinton, Al Gore, Joe Lieberman, John Edwards, and most recently, Barack Obama.

"I don't try to imitate the president, per se," Portman told Yahoo News in an interview. "Just take the policy positions and some of the same rhetoric. It's a good way to learn what the other side is thinking and how they approach problems."

Now, Mitt Romney is said to be considering whether Portman can move from the role of Democratic understudy to Republican vice presidential candidate. If Portman is selected to join Romney's ticket, this time he will have to speak in his own voice.

'He became Barack Obama'

John McCain was preparing for the second national presidential debate in October 2008. Under the rules for the contest, the candidates would sit on waist-high chairs but have the freedom to move around the floor. McCain's advisers were worried about the visual image of Obama, a younger man with a six-inch height advantage, dwarfing the septuagenarian. Portman was instructed to corner McCain on the stage and intimidate him.

"He did it to the point of irritation," Douglas Holtz-Eakin, who served as McCain's chief economic policy director in 2008, told Yahoo News. "Which was the point."

"He was not just the positions of Barack Obama -- he became Barack Obama," Holtz-Eakin said of Portman. "Physical mannerisms, parsing of his voice, everything. He's obviously the white guy from Cincinnati, but Obama has a particular set of vocal rhythms. Rob got all of that."

The work of a debate stand-in can be daunting, given the potential of a particularly good--or bad--debate performance to change the direction of a presidential campaign. There is an exclusive list of surrogates in the political realm with the talent and focus to make the practice sessions truly helpful. The strategy, of course, isn't to engage in "Saturday Night Live"-style mimicry. But the best stand-ins spend days poring over old videos of the candidates they will portray, meticulously studying their policy positions and rhetorical habits.